A helmet generally comprises: a rigid outer shell to prevent direct contact between the user's head and rigid objects and to distribute the impact is force produced thereby over a large surface; and an inner shell of deformable material, typically expanded polystyrene, to absorb said impact force.
To satisfy the requirements of most users, each helmet model is available commercially in numerous different sizes.
The helmet size is a very important parameter in its choice, for reasons both of comfort and of safety. In this respect, the helmet must be able to surround the user's head so that the user's head is unable to move within the interior of the helmet and, especially, so that the helmet is unable to accidentally shift following an impact.
However, notwithstanding the variety of available sizes, the user is able to choose the helmet only within a discrete range of dimensions, which do not always exactly match user requirements.
Moreover for helmet manufacturers, the provision of numerous different sizes for each helmet model involves a considerable multiplication of production and commercial costs.
Helmets with a size adjustment system also exist, composed of an adjustable ring fixed to the helmet interior to surround the head. However, although this known system partially aids size adjustment, the head withdraws, even by a considerable extent, from the inner surface of the protective shell during adjustment, so that when an impact occurs the head itself impacts against this inner surface.